1. Field of the Invention
The present invention relates to a fuel cell system.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. H10-092456 discloses, as a fuel cell system for a small electric equipment, a configuration in which proton exchange membrane fuel cells (or polymer electrolyte fuel cells) are driven with hydrogen supplied from a hydrogen storage alloy and air. The hydrogen releasing process of a hydrogen storage alloy is an endothermic reaction, so that the temperature of the hydrogen storage alloy decreases when supplying the hydrogen fuel. Because the hydrogen releasing ability of the hydrogen storage alloy is lowered with the decrease of the temperature of the alloy, it is necessary to heat the hydrogen storage alloy in order to assure a sufficient flow rate of hydrogen.
On the other hand, because the power generation by means of a fuel cell is accompanied by heat generation, it has been proposed to utilize heat generated by a fuel cell to heat a hydrogen storage alloy. As shown in FIGS. 6 and 7, Japanese Patent Application Laid-Open No. H10-092456 proposes a fuel cell system for a small electric equipment in which a fuel cell stack 1 for generating a power using air and hydrogen, and a hydrogen tank 2 for storing hydrogen to be supplied to the fuel cell stack 1 are connected to each other. The fuel cell system for a small electric equipment has a structure in which the heat generated in the fuel cell stack 1 is guided to the hydrogen tank 2 to heat the hydrogen tank. In the figures, reference numeral 4 denotes an air hole; reference numeral 6 denotes a hydrogen manifold; reference numeral 9 denotes an air blowing means; and the thick solid arrow indicates the movement of air.
However, in the reaction of a fuel cell, hydrogen ions which have passed through a proton exchange membrane (polymer electrolyte membrane) and an oxidizer (oxygen) react with each other in an oxidizer electrode to generate at the oxidizer electrode. Therefore, the exhaust gas of the fuel cell contains water (water vapor). Accordingly, there have been cases where the water-containing gas exhausted from the oxidizer electrode is condensed at the surface of the hydrogen tank to generate water droplets. Especially, in a fuel cell system having a reduced size, the distance between a hydrogen tank and a fuel cell stack is small. Therefore, there has been a problem that when water droplets are generated at the surface of the tank, airflow is blocked thereby making it difficult to supply air necessary for power generation to the fuel cell stack, whereby a stable output cannot be obtained.
The ion conductivity having an influence on the performance of a proton exchange membrane fuel cell greatly depends on the wettability of the proton exchange membrane. That is, drying of the proton exchange membrane leads to remarkable reduction of the conductivity, so that the output is reduced because of the increase of the internal resistance. Accordingly, for power generation using a proton exchange membrane fuel cell, it is necessary that the proton exchange membrane for ionic conduction is suitably wet.
In development of a power supply system using a proton exchange membrane fuel cell, the method of humidifying a proton exchange membrane has been widely considered. For example, Japanese Patent Application Laid-Open No. H09-213359 proposes a fuel cell system composed of a fuel cell stack for generating a power using air and hydrogen, a hydrogen tank for storing hydrogen to be supplied to the fuel cell stack, and a structure for holding water generated in the fuel cell stack and humidifying the hydrogen.
However, as described in Japanese Patent Application Laid-Open No. H09-213359, in the configuration of using the generated water to humidify hydrogen, there has been a problem that a means for introducing the held water into hydrogen gas needs to be separately provided, which increases the size of the fuel cell system.
Further, the configurations of the prior art fuel cell systems have the following problems in attaining a fuel cell system having a more reduced size. That is, when a hydrogen tank is heated using an exhaust gas from an oxidizer electrode of a fuel cell stack, there have been case where the supply of air is blocked by generation of liquid droplets through dewing at the surface of the hydrogen tank. When the flow rate of air is increased in order to prevent the dewing at the surface of the hydrogen tank, there is posed a problem that the proton exchange membrane is dried to reduce the output of the fuel cell. Further, because the power generation makes the temperature of the fuel cell stack higher than the temperature of surroundings, supplying an excessive amount of low-temperature air from ambient air decrease the humidity inside the oxidizer electrode. Therefore, it has been difficult to reduce the size of the fuel cell system while attaining the prevention of dewing at the hydrogen tank and the prevention of drying in the oxidizer electrode at the same time.
Moreover, there has also been a problem that in order to hold the generated water collected at a hydrogen tank or the like and to humidify a supply gas, a separate humidifying mechanism becomes necessary, which makes complicated the configuration/control of the fuel cell system.